The present invention relates to a fuel injection nozzle which injects fuel from an injection hole formed in a nozzle body by the pressure of fuel forcedly fed.
A previous fuel injection nozzle of this type comprises, as shown in FIG. 5, a nozzle needle 105 for opening and closing an injection hole 103 formed in the tip of a nozzle body 121, and a spring 107 for urging the nozzle needle 105 in the direction of closing the injection hole 103. The nozzle body 121 is mounted on a main body 101. In a fuel passage formed in the main body are contained shims 111, 113, a piston 115, a spring seat 117, and a spacer 129, besides the above spring 107. The nozzle body 121 is secured to the main body 101 by a retaining nut 123. On the other hand, in the main body 101 and the nozzle body 121, a fuel passage 125 is formed, and fuel is forcedly fed to the injection hole 103. By the pressure of fuel in the fuel passage 125, the needle nozzle 105 is withdrawn to open the injection hole 103 and to inject fuel from the injection hole 103.
Generally, in this type of fuel injection nozzle, it is effective to restrain the initial injection rate when injecting fuel into a cylinder, for example, in order to decrease NOx and combustion noise of a diesel engine. Therefore, as shown by E in FIG. 5, a clearance for a prelift L1 is defined between the tip portion 127 of the piston 115 receiving a part of pressure (back pressure) in the fuel passage 125 and the spring seat 117. On the other hand, between the nozzle needle 105 and the spacer is established 129, a needle lift L2 (larger than the prelift L1). Because of such arrangements, by the pressure of fuel supplied into the fuel passage 125, the nozzle needle 105 and the spring seat 117 travel through the prelift L1 against the urging force of the spring 107 and a back pressure P applied to the piston 115 in the direction shown by the arrow D1, so that the nozzle needle 105 opens the injection hole 103 (pressure at this time is called the first valve opening pressure) to perform an initial fuel injection at a low injection rate. Thereafter, the nozzle needle 105 and the spring seat 117 further travel through a distance of the needle lift against the urging force of the spring 107 and the back pressure P applied to the piston 115 in the direction of the arrow D1, so that the nozzle needle 105 further opens the injection hole 103 (pressure at this time is called the second valve opening pressure) to perform a stationary fuel injection (main injection) at a high injection rate.
However, the above mentioned previous fuel injection nozzle has the following problems:
(1) When the nozzle needle 105 and the spring seat 117 further travel through the needle lift against the back pressure applied to the piston 115 in the direction of the arrow D1 shown in FIG. 5 by the large pressure of fuel passing through the fuel injection passage 125, the piston 115 releases the shim 111 at the second valve opening pressure and afterward, so that the shim 111 is released from the fixed state and the wear of the shim 111 is liable to occur because of the own vibration of the shim 111. Therefore, when the shim 111 is worn, the tip portion 127 of the piston 115 comes in contact with the spring seat 117 to produce a state where the prelift L1 cannot be ensured. That is, the first valve opening pressure or the initial low injection pressure is lost and a pressure substantially higher than the usual second valve opening pressure comes to be a first valve opening pressure. PA1 (2) Since the state of the shim 111 is unstable, and further, since the piston 115 is supported only on one side by the shim 113, the piston 115 is liable to be inclined from the moving axis for the piston 115, and a cyclic fluctuation in fuel injection performance is liable to occur, and stability thereof is lacking. Consequently, a problem occurs that control of characteristics of fuel injection is difficult. PA1 (3) In a case where the shim 111 is worn and it is necessary to change the shim 111 in adjusting or changing the prelift L1, it is required to remove the nozzle 121, the retaining nut 123, the spacer 129, the spring seat 117, the spring 107, the shim 113, the piston 115, and the shim 111. Consequently, such adjusting operation is very troublesome.
Consequently, delay of fuel injection timing and lowering of the fuel injection quantity in a diesel engine occur. In other words, the characteristics of the fuel injection are changed. Such changing in the characteristics of the fuel injection gives a large load to the diesel engine.